1. Field of the Invention
The present invention relates to the field of semiconductor processing. More particularly, the present invention relates to methods and apparatus for chemically mechanically polishing substrates with increased uniformity and reduced cost. The invention provides apparatus and methods to improve the uniformity of the rate at which material is removed from different locations on the substrate, thereby increasing the number of useful die which are ultimately recovered from the substrate. Additionally, the present invention provides apparatus and methods for simultaneously polishing multiple substrates on a single polishing pad, thereby increasing the productivity of the chemical mechanical polishing apparatus.
2. Background of the Art
Chemical mechanical polishing, commonly referred to as CMP, is a method of planarizing or polishing substrates. CMP may be used as the final preparation step in the fabrication of substrates from semiconductor slices to provide substantially planar front and back sides thereon. CMP is also used to remove high elevation features, or other discontinuities, which are created on the outermost surface of the substrate during the fabrication of microelectronic circuitry on the substrate.
In a typical prior art CMP process, a large rotating polishing pad, which receives a chemically reactive slurry thereon, is used to polish the outermost surface of the substrate. To position the substrate on the polishing pad, the substrate is located in a carrier. The carrier is received on, or directly above, the polishing pad, and it maintains a bias force between the surface of the substrate and the rotating polishing pad. The carrier may also oscillate, vibrate or rotate the substrate on the polishing pad. The movement of the slurry whetted polishing pad across the planar face of the substrate causes material to be chemically mechanically polished from that of the substrate.
One recurring problem with CMP processing is the tendency of the process to over-polishing the planar surface of the substrate, and thereby create localized over-polished and under-polished areas on the substrate. One area on the surface of a substrate where differential polishing commonly occurs is adjacent the substrate edge. When such edge over-polishing occurs, the polished substrate takes on a convex shape, i.e., it is thicker in the middle and thinner along its edge. If the substrate is to be further processed, such as by photolithography and etching, this thickness variation makes it extremely difficult to print high resolution lines on the substrate, Likewise, if CMP is used to remove high elevation features resulting from the formation of circuitry on the working surface of the substrate, differential polishing will physically destroy any die which were formed in the over-polished areas.
Edge over-polishing is caused by several factors. Uneven distribution of the polishing enhancing slurry on the surface of the substrate is one factor which contributes to edge over-polishing. Where the slurry is more rapidly replenished, such as along the edge of the substrate, the substrate is more rapidly polished. Another factor is the relative pressure between the polishing pad and the substrate at different locations on the substrate. The areas where the pressure is higher have higher polishing rates. One relatively high pressure area occurs where the substrate edge presses into the polishing pad, which causes the substrate edge to polish more rapidly than the substrate center. An additional factor, for a polishing apparatus in which the polishing pad and the substrate both rotate, is the cumulative motion between the substrate and the polishing pad. The cumulative motion may be higher near the edge of the substrate than at the substrate center. The greater the cumulative motion between the polishing pad and the substrate, the greater the quantity of material removed from the substrate. As a result of these and other factors, the substrate edge is usually polished at a higher rate than the substrate center.
Substrate over-polishing may also occur in non-contiguous areas of the substrate. This over-polishing is commonly attributed to a warped or otherwise improperly prepared substrate and is exacerbated by the mounting system which affixes the substrate to the carrier. The carrier commonly includes a generally planar lower face face. A conformable material is located on this lower face to receive the substrate there against. The conformable material may be a polymer sheet, or it may be a wax mound over which the substrate is pressed to form a conformable receiving surface. The conformable material, and the lower face of the carrier, may not be as flat as the desired flatness of the substrate. Therefore, the conformable material and generally planar lower face may include protrusions which differentially load the back side of the substrate when the substrate is located on the polishing pad. This differential loading will create overloaded areas on the surface of the substrate engaged against the polishing pad which correspond to the location of the protrusions of the lower face and conformable material. In the localized areas of the substrate where this overloading occurs, the substrate will be over-polished, and the die yield from the substrate will be reduced.
In addition to the reduced die yield which results from the creation of over-polished areas on the substrate, the use of a large rotating polishing pad to sequentially process substrates is inherently inefficient. Typically, the surface area of the substrate is no more that 20% of the surface area of the polishing pad. Therefore, at any point in time, most of the polishing pad material is not in contact with the substrate. One way to increase the utilization of the surface area of the rotating polishing pad is to simultaneously process multiple substrates on the polishing pad. However, users of CMP equipment are reluctant to do so because a substrate may crack or may otherwise be defective, and chips or other contaminants will be transferred by the rotating polishing pad to all of the substrates being simultaneously processed on the polishing pad.
Therefore, there exists a need for a CMP polishing apparatus which provides (i) greater uniformity in the material removal rate between each discrete location or region on the face of the substrate and (ii) greater polishing pad utilization.